Exemplary embodiments of the present invention relate to an injection nozzle for the direct injection of a liquid fuel into a cylinder chamber of a piston engine, a piston engine that is equipped with at least one injection nozzle of this type, and a method for the direct injection of a liquid fuel into a cylinder chamber of a piston engine.
German patent document DE 10 2006 037 413 B4 discloses an injection nozzle that includes multiple spray holes for generating injection jets that extend in a star-shaped pattern, the spray holes being distributed on a nozzle body of the injection nozzle in the peripheral direction. This injection nozzle has six spray holes, which are divided into two groups of three spray holes each. The two groups of spray holes are diametrically opposed. Within the particular group of spray holes, the spray holes adjoining in the peripheral direction enclose essentially the same angle between their longitudinal center axes. This injection nozzle is positioned in the associated cylinder in a targeted manner in such a way that one group of spray holes is associated with two intake valves, while the other group of spray holes is associated with two exhaust valves.
To able to achieve efficient, low-emission combustion in gasoline engines, i.e., spark-ignition piston engines, which operate with formation of an internal mixture, i.e., with direct injection, the fuel must be optimally distributed in the combustion chamber during homogeneous operation. In the process, combustion chamber walls, i.e., a cylinder wall, a top side of the piston facing the cylinder chamber, and a bottom side of the cylinder head facing the cylinder chamber, and the gas exchange valves must be prevented from becoming wetted with liquid fuel, which would result in increased pollutant emissions. This problem is compounded the smaller the size of the available cylinder chambers, since the free propagation paths that are available for the injection jets are correspondingly reduced on account of the reduced displacement. However, engines with increasingly smaller displacements are being sought for reduction of the fuel consumption.
Exemplary embodiments of the present invention are directed to an improved design for an injection nozzle and for a piston engine and for an injection process, which reduces the risk of wetting of combustion chamber walls with liquid fuel.
The invention is based on the general concept of producing an interaction, between two injection jets adjoining in the peripheral direction, which influences the jet propagation direction and thus reduces the penetration depth of the injection jets. The particular interaction results in a deviation of the interacting two injection jets from one another. The invention is based on the finding that this type of interaction with a deviation occurs between two injection jets adjoining in the peripheral direction when the two injection jets are situated at a comparatively small distance from one another in the peripheral direction. The distance from adjacent injection jets may be defined by the angle enclosed by the longitudinal center axes of the two injection jets or enclosed by the longitudinal center axes of the two associated spray holes. The invention makes use of the finding that an individual injection jet interacts with the fresh air enveloping it, such that the fresh air compresses the particular injection jet transversely with respect to its direction of propagation, toward the center of the injection jet, and counteracts an expansion of the injection jet transversely with respect to its direction of propagation. However, if two injection jets are situated relatively close together, only a comparatively small amount of fresh air is present between these two injection jets, so that the above-described interaction with the fresh air is greatly reduced at that location, i.e., at the mutually facing inner sides of the injection jets. As a result, the two injection jets may converge more intensely toward one another. At the same, effective interactions with the fresh air now predominate at the outer sides of the injection jets facing away from one another, so that these divert the two injection jets away from one another. This ultimately results in the desired deviation of the injection jets that are comparatively close to one another.
It is clear that not just the angular distance of the adjacent injection jets is crucial for the desired interaction with deviation of the injection jets. However, this is one of the important parameters. Other parameters influencing the interaction of the adjacent injection jets include, for example, the spray hole length, pressure conditions within the injection nozzle, and the flow pattern of the fuel within the injection nozzle for the particular spray hole.
Furthermore, for achieving the desired interaction with deviation, it is extremely important that within the injection jets at least one pair, composed of exactly two injection jets, is formed, the pair of injection jets adjoining one another in the peripheral direction, and having a distance from one another that is less than the distance of one of the injection jets of the jet pair from another injection jet that is not part of the same jet pair. In other words, the two injection jets associated with the particular jet pair are situated at a comparatively small distance from the associated injection jet of the same jet pair in one direction, while in the other direction, the two injection jets are situated at a greater distance from another injection jet that is adjacent to the jet pair. This ensures that the effective interactions with the fresh air at the outer sides of the two injection jets of the particular jet pair are sufficient to divert the two injection jets away from one another.
For the injection nozzle, this means that at least one pair of spray holes adjoins one another in the peripheral direction, the longitudinal center axes of which enclose a pair angle in the peripheral direction that is smaller than an adjacent angle which is enclosed in the peripheral direction between the longitudinal center axis of each spray hole of the spray hole pair and a longitudinal center axis of a spray hole adjoining the spray hole pair in the peripheral direction.
It has proven to be advantageous to select the respective pair angle to be 50° maximum. The pair angle may be approximately 45°. In contrast, the respective adjacent angle may be 60° minimum. Adjacent angles are preferably approximately 70° or 80°. It is clear that the pair angles and the adjacent angles are basically a function of the number of injection jets or the number of spray holes. The preferred number of spray holes is 5 to 7.
According to another advantageous embodiment, exactly six spray holes may be provided. For six spray holes, exactly two pairs of spray holes may advantageously be provided which are diametrically opposed, a further spray hole being situated in each case between the two pairs of spray holes in the peripheral direction. These two further spray holes are advantageously diametrically opposed. By means of such a configuration it is possible, for example, to associate the two pairs of spray holes with intake valves and exhaust valves in order to achieve reduced penetration depths for the injection jets in the area of the gas exchange valves in a targeted manner. In addition, favorable mixture formations may thus be achieved in the area of an ignition device, provided that one of the pairs of spray holes is associated with the ignition device. A favorable mixture formation improves stratified charge, ignition injection, and catalytic converter heating. At the same time, wetting of the intake valves with fuel may be avoided by aligning the one pair of spray holes with the intake valves.
According to an alternative embodiment in which six spray holes are likewise provided, three pairs of spray holes may be formed which are advantageously uniformly distributed in the peripheral direction, i.e., separated by a distance of approximately 120° with respect to their center axes. The penetration depth for all injection jets may be reduced in such a configuration, so that intake valve wetting may be avoided. In addition, in such configurations a favorable mixture formation may be set in the area of an ignition device. A favorable mixture formation improves stratified charge, ignition injection, and catalytic converter heating.
According to another advantageous embodiment, a supply path leading to the spray holes may be formed in the nozzle body, whereby during an injection process, the fuel flows in this supply path in an inflow direction, which in the area of the particular spray hole in the supply path, is inclined by a range of 60° to 90° with respect to the longitudinal center axis of the particular spray hole. As a result of this measure, an intensive flow deflection is achieved within the nozzle body immediately before the fuel enters into the particular spray hole, which facilitates a reduction in the penetration depth.
According to another embodiment, the particular spray hole may have a hole length and a hole diameter measured coaxially with respect to the longitudinal center axis of the spray hole, a ratio of the hole length to the hole diameter being in a range of 0.5 to 4.0. It has been shown that such ratios of the hole length to the hole diameter facilitate a reduction in the penetration depth.
For example, a hole length may be in a range of 80 μm to 300 μm. Values of 80 μm to 160 μm have proven advantageous for the hole diameter.
The prevailing pressures in the fuel for the injection process may be, for example, in a range of 10 MPa to 25 MPa, i.e., in a range of 100 bar to 250 bar.
In another advantageous embodiment, the particular spray hole may have, in its longitudinal direction, an interior jet formation section and an exterior exit section. The jet formation section defines the hole length and the hole diameter of the spray hole. The exit section has a diameter that is larger than the hole diameter of the jet formation section. The length of the exit section may be essentially equal to the length of the jet formation section. This stepped configuration of the particular spray hole facilitates the generation of injection jets having a small penetration depth.
A piston engine according to the invention includes at least one cylinder and at least one injection nozzle of the above-described type per cylinder, whereby direct injection of liquid fuel into a cylinder chamber of the particular cylinder may be carried out by means of the respective injection nozzle. The injection nozzle is situated in the area of a longitudinal center axis of the cylinder, while an ignition device of the particular cylinder is situated at a distance from the injection nozzle. The particular injection nozzle may now be oriented with respect to its spray holes in such a way that that the ignition device is situated between the two injection jets of such a pair of spray holes in the peripheral direction or between two pairs of spray holes adjoining in the peripheral direction.
A longitudinal center axis of the injection nozzle may extend parallel to the longitudinal center axis of the cylinder. It is likewise possible to provide an inclination, which, however, is preferably less than 20° or equal to 20°, between the longitudinal center axis of the injection nozzle and the longitudinal center axis of the cylinder.
In one advantageous refinement, the ignition device may be situated between two exhaust valves of the particular cylinder in the peripheral direction. Additionally or alternatively, the injection nozzle may be oriented with respect to its spray holes in such a way that the injection jets of such a pair of spray holes extend between two intake valves of the particular cylinder in the peripheral direction. As a result of this orientation, the risk of wetting of the intake valves is significantly reduced.
Further important features and advantages of the invention result from the drawings, and the associated description of the figures with reference to the drawings.
It is understood that the features stated above and to be explained below may be used not only in the particular stated combination, but also in other combinations or alone without departing from the scope of the present invention.